Rigid and Flexible Diaphragm same floor

[coolad2007]

Hi everyone,

I have 4 story (+penthouse) wooden bldg (rectangular in shape 310'x110') which is partially bearing on concrete floor at second level and partially bearing on concrete floor at first level (i.e.) there is a rigid and flexible diaphragm at second floor level. The joint between two diaphragms cuts the bldg into two squares.
The bldg is in washington and hence we need to check it against lateral forces as well. Now my question is,if I dont provide any seismic gap between two diaphragms would the system work.

My main problem is that if I do provide a gap i have to do it on all of the above floors which is not going to happen in our case. I am scared that due to the connection at second level concrete floor might dump the lateral load onto flexible diaphragm whcih will surely fail or the wooden diaphragm might tear out or rupture due to connection to rigid diaphragm.

Any help on the concept of how this bldg will acutally behave would be greatly appreciated.

 

[frv]

The bldg is in washington and hence we need to check it against lateral forces as well

You wouldn't check lateral in Texas?

Anyway.. a sketch would definitely help. In the meantime, always remember that load is "attracted" to stiffness.

 

[msquared48]

With the difference in diaphragm stiffnesses, I would definitely install a seismic separation. Otherwise, you will hear the h@## out of the wood diaphragm.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[dhengr]

As Frv suggested some plans and elevations would be most helpful if you want some meaningful discussion. Also, the stiffer part of the structure will attract more of the lateral loads, in proportion to the relative stiffness of the two parts of the bldg. And, of course, the stiffer part of the bldg. most likely has the greater mass too. I think that the conc. structure might be made to support the wood frame part under the right conditions; it will tend to try, without an intentional separation which will be a nasty detail too. You must also watch out for the way this bldg. will act torsional, in plan, at each level, and special detailing, drag struts and the like, and connections will be reqr’d. You might try to provide some stiff elements at the far end of the bldg., away from the conc. structure part, such as stair or elevator cores. This might start to make things work more practically, reducing the torsional action significantly.

Otherwise, at each level, and at the interconnection btwn. the wood and conc. diaphragms, look at this like the connection btwn. a large roof diaphragm (the wood frame fl.) and a shear wall (the conc. slab), but they are both in the same plane, so details are a bit different. In a typical warehouse bldg. these two diaphragms are much different in rigidity, and also in perpendicular planes, but we make that detail work. You typically might need some drag struts, blocking, slightly stronger diaphragm near the conc. slab, to transfer the wood diaphragm loads and the loads from the wood framed shear walls. These will be regular shear loads and concentrated loads from the torsion action in plan, all taken into the conc. structure, which can then be designed for these added loads.